How the itsy bitsy spider evolved from a giant prehistoric sea creature

It’s hard to believe that a prehistoric sea creature the size of Shaquille O’Neal could teach us anything about a modern dust mite.

But a 7-foot-long, 480-million-year-old marine animal called an anomalocaridid is an ancestor to modern arthropods , the phylum that includes insects, spiders, centipedes, crabs and, yes, dust mites. And a fossil of the species found in the Sahara desert in southeastern Morocco appears to answer a question that’s long puzzled paleontologists: How did arthropods evolve to have legs? The study was released online Wednesday in the journal Nature.

The creatures, among the biggest arthropods that have ever lived, dwelled in the oceans during the Ordovician period, when the seas were rich and teeming with mollusks, tiny fish and sea lilies. The massive supercontinent, Gondwana, was drifting south, and early animals were taking their first limited steps onto land.

It was a time of “massive increases in ecological complexity, with new modes of feeding and complex behavior evolving,” said Peter Van Roy, a paleontologist at Yale University and lead author of the study.

The peaceful, plankton-munching anomalocaridid was abundant in the waters, but until now, finding well-preserved fossils, particularly of the animal’s trunk, has been difficult.

Not long ago, a specimen in good condition was discovered by a local Moroccan collector and moved to Yale’s Peabody Museum of Natural History, where Van Roy spent about 500 hours excavating it from the rock.

Lateral view of a complete specimen of a giant filter-feeding anomalocaridid from the Early Ordovician of Morocco, preserved in three dimensions. Note the presence of two sets of lateral flaps, providing critical new insights into the origins of modern arthropod limbs. Photo by Peter Van Roy, Yale University

As the details emerged, what he saw startled him. The animal, scientists knew, had a head with grasping appendages used to grab prey or filter plankton from the water and flaps on the sides of their long trunks that were used for swimming. But anomalocaridids were believed to have only one set of flaps, and to have completely lacked legs. This one, Van Roy discovered, had two sets of flaps.

“I was literally shocked at the implications of this find ,” he said. “I remember going back to the specimen every day to look at those flaps to say, ‘Yes, they were really there, I’m not seeing things. I’m not going crazy.’” Van Roy and co-authors Allison Daley and Derek Briggs then set out to re-examine other, older anomalocaridids, and found that those too had two sets of flaps, which had been previously overlooked.

When Van Roy first told Daley about the specimen that had just been unearthed, she said she too couldn’t believe what she was hearing.

“I had been working on anomalocaridids for 10 years,” Daley said. “They’re always preserved highly compressed so you don’t get three dimensions. And they all had one pair of flaps per body segments, not two.”

This discovery of two flaps solved the riddle of the supposedly lost legs in anomalocaridids. For without them, how had modern legs of arthropods evolved? The lower set of flaps seem to be modified legs, Van Roy said, used in the case of these animals for swimming. These flaps, Van Roy and his team believe, are the precursors to walking legs while the upper flaps gave rise to gills found in modern anthropods.

David Legg, a paleontologist with Oxford University Museum of Natural History, said the finding is critical to the understanding these animals and their place in evolution.

“Finding legs in this new animal means there isn’t this substantial gap we thought there was … For my own research, it’s quite a big deal.”

And while at first glance, these animals don’t look all that much like a fly or a shrimp, a dust mite or a spider, the similarities jump out at you when you look at the details, Daley said. “The segmentation of the body, the head region, the jointed limbs at the front of the head — these details compare to characteristics of modern arthropods even if they don’t look exactly the same.”

Left:
At seven feet long, the giant, filter-feeding anomalocaridid was one of the biggest arthropods to have ever lived. Illustration by Marianne Collins/ArtofFact